Cellular ceramic electromechanical transducers



June 3, 1959 A. L. w. WILLIAMS ErAL 2,892,

CELLULAR CERAMIC ELECTROMEICI-IANICAL TRANSDUCERS I Original Filed Dec.21,1953 2 Sheets-Sheet 1 PREPARE SLIP OF ELECTROMECHANICALLY ACTIVECERAMIC ADD WATER,WATER-SOLUBLE GELLING AGENT AND WETTING AGENT POURINTO PAPER-LINED WIRE BASKETS; COOL TO SOLIDIFYgDRY.

REMOVE PAPER FROM DRY CERAMIC ELEMENTS AND FIRE ELEMENTS TO MATURITYFIG.Ia

SPRAY SILVER ELECTRODES ONTO OPPOSITE FACES OF CERAMIC ELEMENTS AND FIREAT ABOUT 700C IMMERSE IN TRICHLORETHYLENE AT ABOUT 90C; APPLY POLARIZINGVOLTAGE TO IMMERSED CERAMIC ELEMENTS;

REMOVE POLARIZING VOLTAGE;

REMOVE ELEMENTS FROM TRICHLORETHLENE.'

SEAL POLARIZED CERAMIC ELEMENTS A.L.W.W L F l G BY CHARLES K.GRAVLEYATTORNEY June 23, 19.59 A. L. w. WILLIAMS ETAL I 2, 2,

CELLULAR CERAMIC ELECTROMECHANICAL TRANSDUCERS Original Filed Dec. 21.1953 2 Sheets-Sheet 2 A.L.W.WILLIAMS CHARLES K. GRAVLEY BY ATTORNEYUnited States Patent CELLULAR CERAMIC ELECTROMECHANICAL TRANSDUCERSAlfred L. W. Williams, Cleveland, and Charles K.

Gravley, Willoughby, Ohio, assignors to Clevite Corporation, Cleveland,Ohio, a corporation of Ohio Continuation of abandoned application SerialNo. 399,282, December 21, 1953. This application July 25, 1957, SerialNo. 674,205

8 Claims. (Cl. SID-8.0)

This invention relates to electromechanical transducers, transducermaterials and elements.

The term piezoelectric is used herein as meaning and synonymous to,electromechanically responsive, i.e., capable of converting appliedelectrical energy to mechanical energy or applied mechanical energy toelectrical energy. In other words, these terms are employed hereinwithout distinction as to whether the conversion (or response) is linearor non-linear.

The present invention is concerned primarily with the improvement ofpiezoelectric elements, i.e., electromechanical transducers, composed ofelectromechanically responsive ferroelectric polycrystalline ceramicmaterials which are capable of accepting and retaining electrostaticpolarization. Examples of such transducers are disclosed in UnitedStates Letters Patent No. 2,486,560 to R. B. Gray and No. 2,708,244 toB. Jafie.

The aforementioned patent to Gray discloses transducer elements formedof barium titanate ceramics while the Jatfe patent relates totransducers composed of ceramic solid solutions of lead zirconate andlead titanate. These two are, most likely, the best and most widely usedferroelectric, polycrystalline ceramic transducer materials known at thepresent time and will be used as illustrative examples in describing thepresent invention; however, it will be appreciated as this descriptionproceeds that the basic inventive principles disclosed can be applied toany of the presently known or henceforce discovered electromechanicallyresponsive materials which lend themselves to the performance of themethod steps of the invention. Insofar as is known, only monocrystallinepiezoelectric materials such as quartz, Rochelle salt, ammoniumdihydrogen phosphate, tourmaline and the like would not be satisfactory.

A particularly important aspect of the present invention is inconnection with underwater electroacoustic transducers, in whichelectrical energy applied to the transducer causes it to radiateacoustic energy into the water or an acoustic signal transmitted throughthe water actuates the transducer to produce an electrical response.

It is Well known that a prime requisite of underwater electroacoustictransducers for efficient operation is a good impedance match of thetransducer with water. This applies also to ultrasonic transducers, suchas are used for cleaning, sonic irridation, etc., which operate in fluidtransmission mediums other than water.

The impedance matching of transducers to transmission mediums has longbeen a serious problem in the art. Many of the piezoelectric transducingmaterials and elements heretofore available have a characteristicdensity and mechanical complianceand, therefore, a fixed acousticimpedance. This acoustic impedance usually is much higher than that ofwater or the other transmission fluid involved. This is particularlytrue of the polycrystalline ferroelectric ceramics. For example, thenormal specific acoustic impedances of lead zirconate titanate andbarium titanate ceramics are in the range from about 20 to 30 10 (kg/m?)(m./sec.) as compared to 1.5 X10 for water. Some monocrystallinetransducer materials, e.g., ammonium dihydrogen phosphate (NH H PO haverelatively low acoustic impedance which are a comparatively good matchto water but these materials suffer from other disadvantages: they arelimited in size and, therefore, are not suitable for low frequencyresonant operation unless mass-loading is re sorted to; they arerelatively more expensive to produce than polycrystalline materials andare not susceptible of being formed and shaped by ceramic techniques;and, for generating highly directional signals, large heavy arrays ofmonocrystalline elements must be used because of their individual sizelimitations.

Due to the inherent shortcomings of monocrystalline transducer materialsand elements, the trend in recent years has been toward polycrystallinematerials such as the ferroelectric ceramics mentioned hereinabove.Heretofore, the problem of impedance matching thus encountered has beenattacked by resort to various impedance transformation means. Suchimpedance matching expedients, however, obviously are undesirable inthat they add weight, bulk, complexity and cost to the transducer.

These dilficulties and problems are overcome by the present inventionwhich contemplates an electromechanical transducer comprising amacroscopically porous or spongoid body of electromechanicallyresponsive ferroelectric ceramic material. Due to the fact that theceramic material is macroscopically porous, its density is less and itscompliance greater than conventional material; thus it has a lowercharacteristic impedance which is a much better match to water and mostother common transmission mediums.

It is a general object of the invention to provide electromechanicaltransducers, transducer elements, and materials which overcome at leastone of the problems of the prior art.

It is another general object of the invention to provide a novelferroelectric ceramic element of low density which is capable of asubstantial electromechanical response.

Still another object of the invention is the provision of a noveltransducer element capable of satisfying the prac tical requirements forunderwater transducer operation.

A further object of the invention is the provision of anelectromechanical transducer element of spongiform ferroelectric ceramicmaterial which, because of its porous structure, has a reducedmechanical characteristic impedance, which has a reduced elasticcoupling between its parallel and lateral modes, and which is capable ofan eifective electromechanical response over a wider frequencybandwidth.

A further object of the present invention is to provide more readilymachinable ferroelectric ceramic material adapted for use inelectromechanical transducers.

A still further object of the present invention is to provide a noveltransducer element of ferroelectric ceramic material which has improvedpiezoelectric activity in its parallel mode, as compared with prior artelements of this general type.

Further objects and advantages of the invention as well as the specificdetails of construction and mode of operation of the transducer elementand the preferred manner of making it will be apparent from thefollowing description taken in conjunction with the subjoined claims andannexed drawings, in which,

Figure 1(a) is a flow diagram of the preferred process of makingcellular ceramic transducer materials and elements in accordance withthe present invention;

Figure 1(b) is a flow diagram of further process steps for completingthe fabrication of transducers according to one embodiment of theinvention Figure 2 is a perspective view of a cellular ceramic elementproduced in accordance with the steps in Fig.

Figure 3 is a perspective view of a finished cellular ceramic elementproduced in accordance with the process diagrammed in Figs. 1(a) and1(b); and

Figure 4 is a longitudinal section through an electroacoustie transducerfor underwater operation which incorporates a ceramic piezoelectricelement according to the present invention.

The method for manufacturing the transducers contemplated by the presentinvention comprises two phases: (1) the fabrication of the ceramic bodyor material and (2) the completion of the transducer.

The flow diagram in Figure 1(a) illustrates broadly the steps involvedin the first phase of the method. Thus,

a slip is prepared of the raw ingredients or precursors of a polarizableferroelectric ceramic. To this slip is added water, a gelling agent anda wetting agent. The slip is beaten with a food mixer in a heatedcontainer to aerate it and then pored into paper-lined molds to cool,solidify and dry. The dried elements are removed from the molds andfired to ceramic maturity.

The following examples illustrate the application of the invention tospecific ferroelectric ceramics, viz., barium titanate and leadZirconate titanate.

Example I In practicing the method outlined in Figure 1(a) with bariumtitanate, the first step is to prepare a slip containing, on a weightbasis, barium titanate powder, about 20% or more water, 1 /2 P.V.A.70-05 (polyvinyl alco hol) (another Water-soluble binder such as gelatincould also be used), and 1% Marasperse C.B. (a dispersing agent which isa sodium salt of ligno-sulfonic acid). To this standard BaTiO slip isadded water, Igapal (a wetting agent), 2% triethanolamine (addedprimarily for the purposes of promoting dispersion and plasticizing thebinder so that the resultant elements dry without cracking), and enoughof the water-soluble gelling agents ammonium pentaborate and Congo redthat the resultant elements are stiff at room temperature.

This mixture is a relatively thick gel at this point and is put into acontainer and heated to a temperature of about 55 C. After heating itconverts to a somewhat viscous liquid. The heated liquid mixture then isvigorously agitated to entrain bubbles of air or other ambient gaseousmedium. The aeration may be accomplished conveniently by whipping themixture with a conventional motor-driven food mixer, such as a SunbeamMixmaster. Sufiicient aeration usually requires whipping for eightminutes or more. Use of this method of agitation gives satisfactoryresults with the entrained gas bubbles dispersed more or less uniformlythroughout the mixture. It is pointed out that, in most cases, thewhipping would be carried out in an ordinary atmosphere; however, thiscould be done in an enclosure filled with some other gas and it is to beunderstood that the terms aerated, air bubbles, and the like are usedloosely throughout this description and the appended claims are intendedto encompass gases other than air.

, The density of the finished BaTiO elements produced by the process isdetermined by the amount of water in the BaTiO mixture, the amount ofWetting agent therein, and its temperature during the beating operation.

After being aerated in the manner just described, the foamed BaTiOdispersion is poured into paper-lined, open mesh wire baskets, where theBaTiO is cooled down to room temperature so that it solidifies. Then itis dried thoroughly, which may take from one to three days at roomtemperature and ordinary atmospheric conditions. After having beendried, the BaTiG elements, throughout their bulk, have macroscopicpores, interstices or crevices formed by air bubbles; these elementshave a bulk density of the order of one-fourth. of the theoreticaldensity of barium titanate or the maximum .4 density obtainable as apractical matter in fired barium titanate ceramic.

The paper is removed from the dried BaTiO elements and these elementsare then fired to maturity in substan tially the standard manner ofconventional dense BaTiO; elements, except that in the present processthe firing is carried on at a temperature from about 50 C. to C. higherthan for firing the ordinary dense BaTiO; elements. Accordingly,therefore, in this firing step a temperature within the range from about1380 C. to 1450 C. is maintained.

Example [I In a manner very similar to that described in Example I, themethod was applied to lead titanate zirconate having the formula Pb Sr(Ti Zr )O The composition of a suitable slip of lead zirconate titanateis as follows:

Pb(Zr,Ti)O powder grams 9050 Marasperse do 140 NH OH (Cone. sol.) "cc-..40 Water cc 1600 To 3000 grams of the above slip is added: I

P.V.A. 71-30 (10% so.) grams 200 Congo red do 2 Igapal cc 5 Water cc 300The mixture is treated in accordance with the method steps described inExample 1. However, the firing temperature is adjusted to the materialand for lead zirconate titanate ceramic is about l250-1300 C.

Referring now to Figure 1(b), after firing the ceramic elements tomaturity, they are machined to size by sanding or sawing, and then areelectroded by spraying silver paint onto the opposite major faces of theelements. Preferably the spray is directed at an acute angle to thesefaces so that the electrode material does not penetrate substantiallyinto the interstices of the elements beyond the outer faces thereof. Theelectrode paint is then fired onto the elements at about 700 C. in theusual manner commonly practiced with dense ceramic elements.

Following the electroding operation, the elements are immersed intrichlorethylene at a temperature of about to C. and a relatively highD.C. polarizing voltage applied across electrodes, for example, 15 kv.per inch of thickness.

Finally, foil electrodes having lead-in conductors connected thereto aresecured to the electroded faces of the ceramic element and the elementis sealed against moisture by applying a thermosetting resin to itsexposed edges, as well as to the foil electrodes, if desired.

The transducer element produced by the foregoing process is ofspongiform structure throughout containing separate macroscopicinterstices or crevices filled with air (or other gaseous medium).Because of its cellulated, sponge-like construction the transducerelement has a bulk density which is much lower than the density ofbarium titanate. Depending upon the amount of water added beforestirring and the temperature during stirring, spongoid barium titanatemay have a density within the range from about 0.5 to 3.0, with 1.4being a typical value, as compared with a density of about 5.7 for solidbarium titanate ceramic. Spongoid lead zirconate titanate material mayhave a bulk density within the range from about 1.0 to 4.5 with 2.5being a typical value, as compared with a density of about 7.5 for solidlead titanate zirconate ceramic. Because of the lower density of thespongoid ceramic materials, and because of the higher compliance of theelements due to their cellulated structure, the mechanical (acoustic)characteristic impedance of the spongoid ceramic elements is much lowerthan for dense ceramic elements, this mechanical impedance being makesthem very good transducer elements for underwater operation.

The spongiform transducer elements of the present invention have beenfound to operate effectively over a much Wider frequency band widtharound resonance than has been possible with transducers employing denseceramic elements. Consequently, the transducer elements of the presentinvention are capable of a rapid response to signals which start and endabruptly. Thus, transducers incorporating such elements are particularlywell adapted for echo-ranging using pulse techniques, and otherapplications where a short time constant is vital.

In addition, the cellular ceramic material of the present invention hasbeen found to be considerably easier to machine into a transducerelement of the desired configuration, such as by cutting with a hack-sawor sanding, than is the dense piezoelectric ceramic.

The following table represents a comparison of data on typical samplesof dense, substantially pure, permanently polarized barium titanate anda modified form of lead zirconate titanate with representative samplesof the same materials produced in accordance with the abovedescribedprocess:

BaTiO; Pb,Sr(Zr,Tt)0a* Porous Dense Porous Dense Frequency Constant(para la] mode) 1,150 2, 600 745 1, 970

In the foregoing table: K is the relative dielectric constant orpermittivity with respect to the absolute dielectric constant of freespace; Y is the short-circuited Youngs modulus in the paralleldirection, a ratio of stress to strain, expressed in newtons per squaremeter; 41 is the piezoelectric coefficient relating the parallel strainto the applied electric field, expressed in meters per volt; (1 is thecorresponding piezoelectric coefficient in the lateral mode; g is thepiezoelectric coeflicient, expressed in volt millimeters per newton,which indicates the open circuit electric field strength of the ceramicelement for a given mechanical stress in the parallel mode; k is thecoeflicient of electromechanical coupling in the parallel mode, which isdefined as the ratio of the square root of the mechanical output to thesquare root of the electrical input; and the frequency constant,expressed in kilocycle millimeters, indicates the resonant frequency inkilocycles for a ceramic element 1 mm. thick, this resonant frequencyvarying inversely with the thickness of the element. The parallel, or33, mode refers to mechanical strain in the same direction as theelectric field applied to the ceramic element; in the case of anelectroacoustic transducer element of the expander type, it relates tothe acoustic radiation from either electroded face of the element. Thelateral, or 31, mode refers to mechanical strain perpendicular to theapplied electric field; in an electroacoustic transducer element of theexpander type, it relates to acoustic radiation from any of thenon-electroded faces of a rectangular element.

It will be apparent to those skilled in the art to which this inventionpertains that the present transducer element has a reduced elastic crosscoupling, as a consequence of its cellular structure.

From a comparison of the d coefficients for the respective ceramics inthe above table it will be apparent that there is comparatively littledirect piezoelectric excitation of lateral mode in the cellularsponge-like ceramic Of the present invention. For this reason, when theporous ceramic is operated in the 33 (parallel) mode, radiating acousticenergy from only one electroded face, there is relatively little energyradiated from the element transverse to this direction. Accordingly, theradiated acoustic energy is highly directive and by proper design asubstantially single lobe pattern may be obtained, which is particularlydesirable in certain underwater applications. In the past, because ofthe relatively high elastic cross coupling in dense ceramic elements, itwas not possible to operate such elements in the 33 mode where directivity was an important consideration, except by providing a number ofelements of that type each elongated'in the parallel mode direction andeach having a relatively small radiating face area and arranged inmosaic arrays which were difiicult and expensive to construct foroperation in the desired manner. With the cellular ceramic element ofthe present invention, by proper design good directivity may be obtainedwith an electroacoustic transduced employing a single ceramic elementhaving a relatively large radiating face area which radiates energy inthe 33, or parallel mode. A further important consideration worth notingis that any ceramic element used'for electromechanical transducerpurposes has optimum efficiency when operated in its 33 mode; that is,for a given electrical energy input maximum mechanical output isobtained by operating in this mode. Thus, in the present invention, aceramic transducer element of simple and inexpensive configuration maybe operated in its most efiicient mode (the 33 mode), without resultingin lack of directivity or substantial interference between the paralleland lateral modes.

Figure 2 illustrates an unelectroded, cellular ferroelectric ceramicelement 10 produced by the first five steps of the process described indetail above and outlined in Fig. 1. This element is here shown asrectangular in configuration and in a typical instance may be about 4inches long, by 2 inches wide by about inch thick. As indicated in thedrawing, the element is of cellular cortstruction, having separatedmacroscopic air holes or interstices throughout. I

Figure 3 illustrates this cellular ceramic element after it has beenelectroded and sealed in accordance with the concluding steps in thedescribed process. The interstices throughout the porous ceramic bodyare filled with air. The opposite major faces of the element are coatedwith sprayed-on silver paint against which are secured the foilelectrodes 14 and 15, which may be of brass, or gold-coated silver, orother suitable material. The lead-in conductors 16 and 17 have intimatecontact with the respective foil electrodes. It is intended'to radiateacoustic energy from the major face on the body contacted by electrode14, but not from the other major face at which electrode 15 is located.Acoating 18 of suitable plastic is applied to the outer face of foilelectrode 15. Actually there is a thin layer of air between thiselectrode 15 and the plastic coating and the net effect of thisarrangement is to decouple this'foil electrode 15 from the surroundingmedium, so that no acoustic energy is radiated from this major face ofthe ceramic element. This, of course, does not interfere with theacoustic radiation from the face on the ceramic element contacted byelectrode 14. The moisture-proofing of the ceramic transducer element iscompleted by sealing its edges with a thermosetting resin 19.

In Figure 4, there is shown an underwater transducer employing atransducer element generally similar to that shown in Fig. 3. Theceramic element 20 is identical in all respects to that of Fig. 3,except that the plastic coating on the one electroded face is omitted.Instead, this face, which is contacted by a foil electrode 28, ismounted on a sponge rubber pad 21, which is full of air holes whichacteffectively to decouple this face of the ceramic element. The othermajor face on the ceramic body 20 is contacted by a foil electrode 27,and it is intended to radiate acoustic energy from this face. Themounting pad 21 is mounted on an open-ended housing base 22 across whoseopen end there extends a rubber cap 23. The interior of the housing isfilled with oil. The lead-in conductors 24, 25 for the electrodes on theopposite faces of the ceramic element extend into the housing through afluid-tight seal 26.

In the operation of the transducer for transmitting acoustic energy, avoltage of a predetermined frequency is applied across the electrodes27, 28 on the ceramic element 20, causing acoustic energy to be radiatedfrom the electroded face at 27 of the ceramic element. This acousticenergy is transmitted through the oil and the rubber cap 23 into thewater with very little energy loss therein since both the oil and rubberhave a very good impedance match with water.

Conversely, if the transducer is operated as a receiver, then acousticenergy transmitted through the water passes through the rubber cap 23and the oil in the housing and impinges upon the electroded face at 27on the ceramic element 20, causing the latter to produce a voltageacross the electrodes 27, 28 which is representative of the acousticsignal received.

In the foregoing description, the material of which the transducerelement is composed has been specified as being substantially purepolycrystalline barium titanate or lead zirconate titanate. However, itis to be understood that within the purview of the present invention,transducer elements may be fabricated of other ferroelectric ceramicmaterials which, when polarized, have a substantial electromechanicalresponse, particularly a piezoelectric response. By the term polarized,as used herein is meant either permanently polarized or else subjectedto a temporary polarizing voltage at the time it is operated so as torender it capable of an electromechanical response, particularly apiezoelectric response. As an example of other suitable ceramicmaterial, the ceramic may consist of a mixture of barium titanate and asmall percentage (such as 2% to 3 /z%) of zirconia (ZrO or bariumtitanate and a small percentage of barium zirconate (BaZrO as disclosedand claimed in U.S. Patent No. 2,708,243 to E. I. Brajer. Alternatively,other mixtures of barium titanate, or piezoelectric ceramic materialsother than barium titanate, may be used. Additional examples of suitablematerials are given in copending applications Serial No. 527,720 filedAugust 11, 1955, and Serial Nos. 550,868 and 550,869 filed December S,1955.

Insofar as the transducer element itself is concerned,

without departing from the purview of this invention it may be made byprocesses other than that described herein, so long as it has the lowdensity, cellular structure which renders it capable of accomplishingthe purposes of this invention.

Therefore, while there have been disclosed in the fore- ,goingdescription a specific presently preferred manner ofpracticing theprocessof the present invention and a specific preferred embodiment ofthe ceramic transducer element itself, it is to be understood thatvarious modifications, omissions and refinements which depart from thedisclosed embodiments of the process and product of the presentinvention may be adopted withoutdeparting from the spirit and scope ofthis invention.

This application is a continuation of Serial No. 399,282 filed December21, 1953, and subsequently abandoned.

We claim:

1. An electromechanical transducer element comprising a body of spongoidstructure formed with macroscopic interstices throughout, said bodyconsisting essentially of ferroelectric ceramic material capable of asubstantial electromechanical response.

2. An electromechanical transducer element in the for of a firedspongoid body consisting of polycrystalline ferroelectric ceramicmaterial capable of a substantial piezoelectric response having asubstantially lower bulk density than theoretical density of saidmaterial.

3. An electromechancial transducer element comprising a body ofpolarizable ferroelectric material of macroscopic porosity.

4. An electromechanical transducer element according to claim 3, whereinsaid material is composed primarily of barium titanate.

5. An electromechanical transducer element according to claim 3, whereinsaid material is composed primarily of lead zirconate titanate.

6. An electromechanical transducer element comprising an aerated bodyconsisting primarily of polycrystalline ferroelectric ceramic materialpermanently polarized in one direction and of spongiform structurethroughout, the bulk density of said body being substantially lower thanthe theoretical density of said ceramic material, and electrodes inintimate contact with a pair of opposite faces on the body.

7. An electromechanical transducer element according to claim 6 whereinsaid pair of faces extend perpendicular to the direction ofpolarization.

8. An electromechanical transducer element in the form of a firedmacroscopically porous ceramic body consisting primarily of apolarizable ferroelectric ceramic material selected from the groupconsisting of barium titanate and lead zirconate titanate and having adensity within the range from about 0.5 to 4.0 grams per cc.

References Cited in the file of this patent UNITED STATES PATENTS

